1. Field of the Invention
The present invention relates to a magneto-optical recording medium for recording, reproducing and erasing information by a laser beam.
2. Description of the Prior Art
As a computer technology has been developed, a huge volume of information must be processed. A high density and high capacity optical memory is one of many technical factors which support such information processing.
Practical recording methods now available include a method for irradiating a laser beam onto a record disk to form pits and a method for changing an optical characteristic such as a reflection factor.
However, those methods are limited to reproduction only or addition of recorded information. A medium which allows erasure of information and recording of new information has not been available.
The present invention relates to a magneto-optical record medium which can be used as an element to allow recording, erasing and recording of information.
Reproducing methods of a magneto-optical record medium include a method which utilizes a Faraday effect and a method which includes a Kerr effect. The magneto-optical recording medium which is used in those methods has an advantage described above but a disadvantage of low reproduced signal level. In the Kerr effect reproduction system, a high signal to noise ratio (S/N) is difficult to attain because of a small Kerr rotation angle. In order to increase the Kerr rotation angle, a magnetic material must be improved or a dielectric thin film such as SiO or SiO.sub.2 is formed on the record medium to utilize multiple reflection onto a magnetic layer. As shown in Japanese Patent Application Laid-Open Nos. 6541/1980 and 6542/1983, an amorphous thin film is used and a metal reflection layer is formed on a backside so that the Kerr effect and the Faraday effect are utilized to increase the Kerr rotation angle. It is reported that the Kerr rotation angle is increased by forming a transparent dielectric film between the amorphous magnetic layer and the reflection layer with optimum selection of the materials and the thicknesses of those three layers. (J. Appl. Phys. Vol. 53 No. 6 pages 4485-4494).
However, in those methods, the film thickness of the amorphous magnetic film must be less than 250 .ANG. in order to allow transmission of a readout light. If the record medium is left in a high temperature and high humidity environment in the presence of oxygen, the amorphous magnetic layer is readily oxidized and this results in reduction of a recording sensitivity of the medium, increase of error in recording and reproducing information and degradation of the signal.